1. Field of the Invention
The present invention relates generally to an antenna, and more particularly to a dual-band antenna for use with a wireless communication device.
2. Description of the Prior Art
With the development of wireless communication technology, various products such as mobile computers for dual-band communication have high performance to meet the consumers' demands. Accordingly, if a mobile computer with wireless communication function desires to have high performance, it is critical for it to have a well-designed antenna, which having high gain, high directivity when required and characteristics that can be applied in dual frequency bands.
Conventional antennas generally adapted to wireless communication products such as mobile computers are substantially grouped into two types, wherein one is external antenna and the other is internal antenna. The external antenna protrudes a relatively long distance from the body of the mobile computer, which makes the computer aesthetically unpleasing and inconvenient to move. In addition, the antenna is often bent, broken, knocked out of alignment or otherwise damaged because they can easily catch or strike objects such as people, walls, doors, etc. Furthermore, the antenna requires a large support structure to secure the antenna to the housing of the computer and this support structure requires a considerable amount of space inside the body of the computer. This space is valuable, especially in small, portable computer.
Accordingly, in order to clear said problem of the external antenna, various kinds of internal antennas dedicated to mobile computers are extensively used, which include slot antennas, microstrip antennas, planar inverted-F antennas (PIFA), spiral antennas and so on. A typical conventional planar inverted-F antenna (PIFA) is disclosed in U.S. Pat. No. 6,600,448 issued to Ikegaya et al on Jul. 29, 2003. The antenna provided in the Ikegaya's patent is a thin flat-plate antenna having a slit, which has a specified width and a specified length and is formed in a conductive flat plate. A radiating element portion shaped like a monopole antenna and a ground portion are formed with the slit between them. Although, the planar inverted-F antennas (PIFA) are structured so compact and lightweight, it can only operate in a single frequency band, which limits the use of this conventional antenna. Therefore, it is expected to develop an antenna adapted for dual frequency bands along with the mainstream trend of related communication device. For example, U.S. application Ser. No. 10/330959 filed by the same applicant discloses a dual-band antenna, which is able to operate in dual frequency bands (such as 2.4 GHz and 5.2 GHz) and has a compact shape particularly adapted to the communication products such as mobile computers. Horizontal portions of this antenna separated from each other serve as radiating element portions and a ground portion, respectively. A connection strip with respect to a feeding point thereof links the radiating element portions and ground portion. A coaxial feed cable is soldered onto the connection strip. The impedance matching between the antenna and the coaxial cable is realized by moving the feed point at the connection strip of the antenna. However, this means is limited by physical dimensions of connection strip and may influence the resonant frequencies of the antenna. Consequently, how to choose the feed point, which makes the antenna attain impedance matching and desired resonant frequencies as well, is relatively concerned.
Hence, it is necessary to provide a build-in antenna, which is capable of operate not in less than dual frequency bands and can easily achieve impedance matching between the antenna and the coaxial cable.